This invention relates to an electroluminescent (EL) lamp and, in particular, to an EL lamp displaying a graphics image which is produced by controlling the electric field between the electrodes of the EL lamp.
An electroluminescent (EL) lamp is essentially a capacitor having a dielectric layer between two conductive electrodes, one of which is transparent. The dielectric layer may include a phosphor powder or there may be a separate layer of phosphor powder adjacent the dielectric layer. As used herein, the term "electroluminescent dielectric layer" includes both constructions. The phosphor powder radiates light in the presence of a strong electric field, using very little current. The front electrode is typically a thin, transparent layer of indium tin oxide or indium oxide and the rear electrode is typically a polymer binder, e.g. polyvinylidene fluoride (PVDF), polyester, vinyl, or epoxy, containing conductive particles such as silver or carbon. The front electrode is applied to a polymer film such as polyester or polycarbonate to provide mechanical integrity and support for the other layers.
It is often desired to have an EL lamp produce a graphic image when illuminated, e.g. the numerals in a watch face, a corporate logo or other symbol, or text. These graphics can be produced by patterning one or both electrodes of the EL lamp, forming gaps in the electrodes. Since the lamp operates by virtue of an electric field across the electroluminescent dielectric layer, there must be contact to the electrode over any area which is to be luminous and the bridge between luminous areas is itself luminous. The result is that closed figures, such as a circle, are very difficult to produce and alphanumeric characters appear stenciled. Even if an appropriate design can be made without closed figures, the gap between portions of the electrode produces an undesirable dark line that is often visible even when the lamp is not luminous.
EL lamps having a segmented electrode are known in the art. For example, U.S. Pat. No. 3,813,575--Webb--discloses an EL lamp having a single transparent electrode and a segmented rear electrode. The EL lamp includes seven segments for representing a single digit in an alphanumeric display and each digit requires seven contacts, plus one contact for the front electrode. Providing space for and locating contact areas is often difficult, particularly in applications where space is at a premium such as in a watch face. A minimum number of contacts is preferred.
U.S. Pat. No. 2,928,974--Mash--discloses an EL lamp having a split rear electrode to which the leads of the lamp are attached. The applied voltage is capacitively coupled to the front electrode and the lamp is equivalent to two capacitors in series. Japanese Patent 5-283164, issued Oct. 29, 1993, also discloses an EL lamp having a split rear electrode. A split electrode reduces the number of contacts but raises the voltage necessary to drive an EL lamp to the desired brightness.
A problem with a split rear electrode is that the lamp segments must be of equal area in order to have the same brightness. Obviously, this severely limits the complexity of the graphic. An alternative is to separately power each lamp segment, which would increase the number of contacts and raise the capacitance of the load on a power supply for the lamp segments.
A problem with patterned electrodes is that positive and negative graphics cannot be produced with equal ease. For example, if text is displayed as dark-on-light, then the background is a single lamp. If the same text is displayed light-on-dark, then each character of text is a separate lamp and must be individually connected to a source of power (otherwise the brightness of the letters varies with their area). Thus, inverse or negative graphics are difficult to obtain. This can become particularly troublesome if the reverse of a corporate logo is not a photographic negative (a simple reversal of light and dark); i.e. either version of the logo may require a plurality of individual lamps.
A graphic can be added to an EL lamp by printing opaque material on the outer or front surface of the lamp, overlying the transparent electrode. A problem with this construction is that the graphic is always visible. Many customers for EL lamps want a graphic visible only when the lamp is lit.
In view of the foregoing, it is therefore an object of the invention to provide an EL lamp which can produce complex graphic images and can be constructed with continuous electrodes, i.e. with electrodes which are not patterned or segmented.
Another object of the invention is to provide an EL lamp which can display a graphic including intermediate brightness levels as determined by the desired graphic, i.e. the lamp can produce a gray scale.
A further object of the invention is to provide an EL lamp which can produce shades of gray independently of the area of each shade.
Another object of the invention is to provide an EL lamp in which separate lit areas have the same brightness, regardless of area.
A further object of the invention is to provide an EL lamp having continuous electrodes and areas of different brightness.
Another object of the invention is to provide an EL lamp which displays a graphic only when lit.
A further object of the invention is to provide an EL lamp which can produce positive and negative graphics with equal ease.